1. Field of the Invention
The present invention relates to systems for eliminating arcing faults at components of electric power systems and, more particularly, to such systems for detecting and eliminating arcing faults within the metal cabinets of switchgear. The invention also relates to power distribution systems employing shorting switches for eliminating arcing faults.
2. Background Information
Electric power systems incorporate switches for control and protection purposes. Distribution systems, which form part of the overall electric power system, include main and branch power buses and circuit breakers mounted in metal cabinets to form switchgear. Interruption of current flow in the buses of the distribution system by a circuit breaker creates an arc as the contacts of the circuit breaker open. These arcs caused by interruption are contained and extinguished in the normal course of operation of the circuit breaker.
At times, however, unintended arcing faults can occur within switchgear cabinets, such as between power buses, or between a power bus and a grounded metal component. Such arcing faults can produce high energy gases, which pose a threat to the structure and nearby personnel. This is especially true when maintenance is performed on or about live power circuits. Frequently, a worker inadvertently shorts out the power bus, thereby creating an arcing fault inside the enclosure. The resulting arc blast creates an extreme hazard and could cause injury or even death. This problem is exacerbated by the fact that the enclosure doors are typically open for maintenance.
A common approach to protecting personnel from arcing faults in switchgear has been to design the metal enclosures to withstand the blast from the arcing fault. This has been done at great additional costs due to the heavy gauge metal used and numerous weld joints needed to prevent flying debris. Even with these precautions, the blast from an arcing fault inside the switchgear cannot be contained.
Recently, methods have been developed to minimize the severity of the blast from an internal arcing fault. These methods include pressure sensing and light detection, which sense the arcing fault within the switchgear and cause a circuit breaker to trip before significant damage can result. The pressure sensing method is limited by the insensitivity of the pressure sensors. By the time cabinet pressure has risen to detectable levels, the arcing fault has already caused significant damage.
In a medium voltage system, an internal arcing fault would occur somewhere inside of the switchgear enclosure, frequently, but certainly not limited to the point where the cables servicing the load are connected.
In a low voltage system, such as, for example, a motor control center, an internal arcing fault could occur within the load center panelboard when, for example, servicing line panelboards. A bare live copper bus could inadvertently be shorted. Another example for both low and medium voltage systems would be the shorting of the conductors by rodents, snakes, or other animals or objects.
In the low voltage system, the arcing fault could clear itself, by burning or ejecting the short, but it may take more than one-half cycle to do so, thereby causing significant damage and great risk of injury to workers even in one-half cycle of arcing. Thus, the need for a sub-one-half cycle high-speed switch.
A medium voltage system would behave similar to the low voltage system; however, the medium voltage system would be less likely to be self-extinguishing. The crowbarring of the shorting switch will extinguish the arc. Once the arc is out, and if the short has been burned away or removed, then system power can be restored.
It is known to employ a high-speed shorting switch to eliminate an arcing fault. Known arc elimination devices and systems produce a bolted fault across the power bus (e.g., phase to phase, such as two switches for three phases; phase to ground, such as three switches for three phases), in order to eliminate the arcing fault and prevent equipment damage and personnel injury due to arc blasts. It is also known to employ various types of crowbar switches for this purpose. The resulting short on the power bus causes an upstream circuit breaker to clear the bolted fault by removing power. See, for example, U.S. Pat. Nos. 6,633,009; and 6,657,150. As a result, system power is lost due to the tripping of the upstream circuit breaker.
Such arc elimination devices and systems may be applied in low voltage (e.g., up to about 690 VAC) and/or medium voltage (e.g., about 1 kV to about 38 kV) applications. For example, FIG. 1 shows medium voltage (e.g., 15 kV/60 MVA with a 50 kA fault potential) switchgear 2 for a three-phase power source 4. Associated with a three-phase power bus 6 is a first shorting switch 8, which is disposed between phases A and B, and a second shorting switch 10, which is disposed between phases B and C. Although the three-phase switchgear 2 and power source 4 are shown, one of the shorting switches 8,10 may be applied in a single-phase application (not shown). Although phase to phase shorting switches 8,10 are shown, such shorting switches may be applied from phase to ground 12. Disposed within the switchgear 2 are a plurality of light sensors 14,16,18,20, which detect the presence of arc light 22 associated with an arcing fault 24. In response to the arcing fault 24, one or more of the sensors 14,16,18,20 detect and communicate the presence of the arc light 22 to a trigger/power circuit 26, which responsively sends an actuation signal 28 to one or both of the shorting switches 8,10.
Shorting switches, however, shut down the system fed by the associated switchgear. Loss of power, even for a few seconds, can be devastating for critical loads (e.g., relatively large motors) and critical processes or operations (e.g., power plants; petrochemical plants; emergency backup paper mills).
There is a need, therefore, for improved apparatus and method for detecting and clearing arcing faults in electric power systems and, particularly, within switchgear.
Accordingly, there is room for improvement in systems for eliminating arcing faults and in power distribution systems employing shorting systems.